1. Field of the Invention
This invention relates to light beam illumination systems, and more particularly relates to method and apparatus for producing a light beam of selected cross-section shape and uniform intensity, and which emits self-luminously into a selected numerical aperture.
2. Description of the Prior Art
Precision illumination systems are used extensively in the production of integrated circuit chips and electronic circuit boards. Such illumination systems typically include a primary light source such as a high intensity lamp or a laser; a target positioning system; a protection system to illuminate the target with the selected pattern; and a control system. The intent typically is to illuminate a chip covered with a photoactive layer so as to produce the desired circuit pattern, which later will be metallized or otherwise activated during further processing. Illumination may be by ultraviolet light, visible light, electron-beam or a combination spectrum of radiation. The desire is to illuminate the target chip selectively, so as to activate a particular pattern. Integrated circuit chips typically undergo several illumination steps during production.
The high resolution illumination systems used in such applications as semiconductor integrated circuit production, and in production of electronic circuit boards, use or high-intensity lamps or lasers as producers of primary light beams and may employ lens systems to expand the primary beams into beams of the desired shape and intensity. The expanded light beams may however not be of uniform intensity when expanded. If derived from a laser source or via collimating lens systems, the beam may be collimated and thus lack the self-luminance property which many applications demand.
Uniformation of the light beam transverse intensity distribution has in the prior art been approached by such light scattering artifices as a fly's-eye lens, a diffuser or a rotating mirror.
Self luminance is the characteristic of uniformity of multi-directional light emissivity at all points across a light beam.
The self-luminance property of an illumination system is such that any point across the beam emits into a range of directions, in a core which may be described in terms of a numerical aperture. Collimated light, that is, light which is not scattered but in which each ray is parallel to the beam axis, provided when a laser is the primary light source. A collimated light beam from a laser is not self-luminous. Light from certain high-intensity lamps may be collimated be associated reflection and lens systems and thus is not self-luminous.
The varying demands of industrial applications of illumination systems are best served by an illumination systems which efficiently produces a light beam of desired wavelength and cross-section shape, having the property of uniform intensity distribution across the cross-section, and having the self-luminance property with a desired numerical aperture of emission. These demands have typically been approached by using a primary light source subsystem which is lacking in uniformity, and providing uniformization in the light beam characterization subsystem through some artifice such as a fly's-eye lens, diffuser or rotating mirror.
An approach of the prior art is to use some efficient source of light, such as a halogen lamp with optics, or a laser, as a primary light source. This typically produces a collimated light beam. Typically, a diffuser plate etched to a degree of roughness is used to provide the desired uniformity property.
Another approach of the prior art is to use a fly's-eye lens to provide the uniformity property to the beam. The fly's-eye lens is a multiple lens array, which essentially replicates the scene hundreds of times, and can thus scatter the light so as to replicate the point source hundreds of times and thus provide the uniformity property. Note that the fly's-eye lens is thus essentially used as a diffuser.
Another approach of the prior art is to use a concentrated light beam from a primary source, but to scan the concentrated light beam across the desired cross-section by using a rotating mirror.
The artifices used to provide the self-luminous property have tended to reduce efficiency. There has been a continuing need for improvement in illumination systems.